Download Weathering and Soil Formation

Weathering and Soil Formation
Table of Contents
• Rocks and Weathering
• How Soil Forms
• Soil Conservation
Weathering and Soil Formation - Rocks and Weathering
Weathering and Erosion
• Weathering – the chemical and
physical processes that break
down rock at Earth’s surface.
– Agents of weathering include:
Heat, cold, water, ice, and
gases in the atmosphere
• Erosion – process by which
water, ice, wind, or gravity moves
weathered rock and soil.
• Weathering and erosion work
together continuously to wear
down and carry away the rocks
at Earth’s surface.
Weathering and Soil Formation - Rocks and Weathering
Mechanical Weathering
• There are two kinds of weathering:
mechanical weathering and chemical
weathering.
• Mechanical weathering – type of
weathering in which rock is physically
broken into smaller pieces.
– The smaller pieces of rock have the
same composition as the rock they came
from.
– A very slow process, but eventually can
wear even the tallest mountains down
Weathering and Soil Formation - Rocks and Weathering
Mechanical Weathering
– Causes of mechanical weathering
include:
•
freezing, thawing, release of
pressure, plant growth, actions of
animals, and abrasion.
• Abrasion - the grinding away of rock by
rock particles carried by water, ice, wind,
or gravity.
• Ice wedging – process that splits rock
when water seeps into cracks, then
freezes and expands.
Weathering and Soil Formation - Rocks and Weathering
Chemical weathering
• Chemical weathering –
process that breaks down rock
through chemical changes.
– Causes of chemical weathering
include:
• action of water, oxygen, carbon
dioxide, living organisms, and
acid rain.
– Creates holes or soft spots in
rock, causing the rock to break
apart more easily.
Weathering and Soil Formation - Rocks and Weathering
• As weathering breaks apart rock,
the surface area exposed to
weathering increases.
Weathering and Soil Formation - Rocks and Weathering
Chemical weathering
• Water - weathers rock by dissolving it.
• Oxidation – when a substance combines
with oxygen a chemical reaction occurs.
– Ex: Iron + Oxygen = Rust
• Carbon Dioxide - dissolves in rainwater
and creates a weak acid called carbonic
acid that breaks down rocks.
Weathering and Soil Formation - Rocks and Weathering
Chemical weathering
• Living Organisms – roots from plants
push into cracks of rocks and produce a
weak acid.
• Acid Rain – the result of gases from
burning fossil fuels combining with water
vapor (clouds) creating weak acid
droplets.
Weathering and Soil Formation - Rocks and Weathering
Which Weathered Faster?
• The graph shows the
rate of weathering for
two identical pieces
of limestone that
weathered in
different locations.
Weathering and Soil Formation - Rocks and Weathering
Which Weathered Faster?
• Reading Graphs:
What does the x-axis
of the graph
represent?
– Time in years
Weathering and Soil Formation - Rocks and Weathering
Which Weathered Faster?
• Reading Graphs:
What does the y-axis
of the graph
represent?
– The thickness of
stone lost to
weathering
Weathering and Soil Formation - Rocks and Weathering
Which Weathered Faster?
• Reading Graphs:
How much thickness did
Stone A lose in 1,000
years? Stone B?
– Stone A lost about 8.5
millimeters; Stone B
lost slightly more than
4 millimeters.
Weathering and Soil Formation - Rocks and Weathering
Which Weathered Faster?
• Drawing Conclusions:
Which stone
weathered at a faster
rate?
– Stone A weathered
at a faster rate.
Weathering and Soil Formation - Rocks and Weathering
Which Weathered Faster?
• Inferring:
Since the two identical
pieces of limestone
weathered at different
rates, what can you
infer caused the
difference in their rates
of weathering?
– They were exposed to
different climate
conditions.
Weathering and Soil Formation - Rocks and Weathering
Rate of Weathering
• The most important factors that
determine the rate at which weathering
occurs are the type of rock and the
climate.
• Type of rock – the mineral composition
determines how fast a rock weathers.
– Minerals that do not dissolve easily in
water weather slowly.
– Permeable rocks – contain tiny,
connected air spaces that water can
seep through.
• Causes faster weathering
More surface
area
Weathering and Soil Formation - Rocks and Weathering
Rate of Weathering
• Climate – both chemical
and mechanical
weathering occur faster in
wet climates.
– Chemical reactions also
occur faster at higher
temperatures.
• Tropical rainforests have
the highest rate of
weathering.
Weathering and Soil Formation - Rocks and Weathering
More on Weathering
• Click the PHSchool.com button for an
activity about weathering.
Weathering and Soil Formation - How Soil Forms
What Is Soil?
• Soil – loose, weathered
material on Earth’s surface
in which plants can grow.
– Created from bedrock below
• Bedrock – the solid layer of
rock beneath the soil.
Weathering and Soil Formation - How Soil Forms
What Is Soil?
• Soil is a mixture of rock particles, minerals,
decayed organic material, water, and air.
– sand, silt, and clay make up the portion of soil
that comes from weathered rock.
– Humus – a dark-colored substance that forms
as plant and animal remains decay.
• Provides nutrients for plants to grow
– Fertility of soil – a measure of how well the
soil supports plant growth
• Soil rich in humus has a higher fertility
Weathering and Soil Formation - How Soil Forms
Soil Texture
• Soil texture depends on the size of the soil particles.
– Gravel – largest > 2mm
– Sand – between 1/16 – 2 mm
– Silt – between 1/256 – 1/16 mm
– Clay – smallest < 1/256 mm
Weathering and Soil Formation - How Soil Forms
What Is Soil?
• Loam – type of soil,
is made up of air,
water, and organic
matter as well as
materials from
weathered rock.
– Best for growing
most types of
plants.
Weathering and Soil Formation - How Soil Forms
The Process of Soil Formation
• Soil forms when
rock is broken
down by
weathering and
mixes with other
materials on the
surface.
• Soil is constantly
being formed
wherever bedrock
is exposed.
Weathering and Soil Formation
The Process of Soil Formation
• Soil horizon – layer of soil that differs in
color and texture from the layers above or
below it.
– Topsoil – mixture of humus, clay, and other
minerals that forms the crumbly, topmost layer of
soil. (Horizon A)
– Subsoil – layer of soil beneath the topsoil that
contains mostly clay and other minerals.
(Horizon B)
– Horizon C – contains only partly weathered rock
Weathering and Soil Formation - How Soil Forms
Soil Layers Activity
• Click the Active Art button to open a
browser window and access Active Art
about soil layers.
Weathering and Soil Formation - How Soil Forms
Soil Types
• Scientists classify the different types of soil into
major groups based on climate, plants, and soil
composition.
Weathering and Soil Formation - How Soil Forms
Living Organisms in Soil
• Some soil
organisms make
humus, the
material that
makes soil fertile.
Other soil
organisms mix
the soil and make
spaces in it for air
and water.
Weathering and Soil Formation
Living Organisms in Soil
• Most humus is formed from plant litter.
– Litter – loose layer of dead plant leaves and
stems on the surface of the soil.
• Humus forms from decomposition
– Decomposers – organisms in the soil that
breaks down the remains of dead organisms
and digests them.
– The remaining organic material is put back
into the soil.
Weathering and Soil Formation
Living Organisms in Soil
• Earthworms are
responsible for mixing
up the soil and making it
rich for plants.
• Many burrowing
mammals such as mice,
moles, prairie dogs, and
gophers also break up
hard, compacted soil
and mix humus through
it.
Weathering and Soil Formation - How Soil Forms
Soil
• Click the Video button to watch a movie
about soil.
Weathering and Soil Formation - Soil Conservation
Value of Soil
• Natural resource – anything in the
environment that humans use.
• Soil is one of Earth’s most valuable natural
resources because everything that lives on
land, including humans, depends directly
or indirectly on soil.
Weathering and Soil Formation - Soil Conservation
Soil Damage and Loss
• Wind erosion is one cause
of soil loss.
• Ex: Wind erosion created
the Dust Bowl on the Great
Plains.
• Farmers removed grasses
containing sod causing soil
to dry and blow away
• Sod - thick mass of tough
roots at the surface of the
soil that keeps soil in place
Weathering and Soil Formation - Soil Conservation
Soil Damage and Loss
• Read pages 56-58 on
“The Dust Bowl”
– While reading think
about these
questions:
• Could this problem
happen again?
• How would you be
affected?
Weathering and Soil Formation - Soil Conservation
Soil Conservation
• Soil conservation – the management of
soil to prevent its destruction.
– These methods include: contour plowing,
conservation plowing, and crop rotation.
• Contour plowing – plowing fields along
the curves of a slope to prevent soil loss.
Weathering and Soil Formation
Soil Conservation
• Conservation plowing – dead stalks from
the previous year’s crop are left in the
ground to hold the soil in place.
• Crop rotation – planting of different crops
in a field each year to maintain the soil’s
fertility.
Weathering and Soil Formation - Soil Conservation
Links on Soil Conservation
• Click the SciLinks button for links on soil
conservation.
Weathering and Soil Formation - Soil Conservation
Soil Conservation
• Click the Video button to watch a movie
about
soil conservation.
Weathering and Soil Formation
Soil Horizons
Bedrock begins to weather
The C horizon forms as bedrock breaks apart.
The A horizon develops from the C
horizon where plants grow in it.
The B horizon develops as clay and minerals
wash down from the A horizon.